1. Field of the Invention
The present invention relates to the field of fuel injectors, fuel injection systems and methods of operation thereof.
2. Prior Art
Fuel injector performance, particularly in diesel engines, has a substantial influence in overall engine performance, especially with respect to emissions. Of particular importance is the speed at which fuel injection can be terminated. In particular, if fuel injection is terminated merely by the reduction in injection pressure it is difficult to rapidly terminate injection because of the compressibility of the fuel and actuation fluid in an intensifier type fuel injector, resulting in a trail off in atomization resulting in unacceptable levels of unburned fuel in the exhaust. Accordingly various types of direct needle control have been proposed to provide injection control other than by controlling injection pressure.
Also fuel injectors, particularly diesel fuel injectors, are using ever increasing injection pressures, now going as high as 3000 bar (45,000 psi). Diesel fuel has a compressibility of approximately 1% per 67 bar (1000 psi), so that at the injection pressure, the fuel has been substantially compressed. In intensifier type fuel injectors, injection occurs directly as a result of intensification, so that injection begins on intensification and terminates on termination of intensification. Consequently the volume of fuel intensified is set equal to the maximum injection volume needed, plus of course some overhead volume for the needle chamber, passageways to the needle chamber, etc. At a partial power setting for the engine, much less than the maximum injection volume is needed, yet the full amount is compressed and then depressurized, losing the energy required for the compression of the fuel not injected, which at low power settings and at idle, can be most of the substantial amount of energy used for intensification. In fuel injectors having direct needle control, the operation is a bit different, in that intensification occurs, then injection by the direct needle control, then termination of injection, again by direct needle control, and then depressurization to refill the intensification chamber for the next cycle. While this cycle is a bit different, the losses of intensification energy are not different.
Injectors using direct needle control to control injection of fuel supplied to the injector at injection pressure from an external source are also known. These injection systems are more efficient because fuel, once compressed, is sooner or later all injected regardless of the engine power setting. They also have the advantage of not cycling the fuel pressure in the needle chamber on each injection event, helping reduce, but not eliminate, the possibility of eventual injector tip breakage. However such systems have serious drawbacks. Aside from the safety issues of having a rail at injection pressures and the associated plumbing problems, there is a serious risk to the engine, in that if an injection tip breaks off, a direct and continuous flow path from the high pressure rail to the combustion chamber is provided, which could result in a hydraulic lock of the engine with catastrophic results.
Also known are methods of operation of intensifier type fuel injectors whereby a quantity of fuel is intensified by an intensifier and injection is controlled by direct needle control with the intensifier still being active until the remaining intensified fuel is less than needed for the next injection event or a portion of an injection event, at which time the intensifier is deactivated to refill the intensifier with fuel. Then the intensifier is activated again for subsequent injection events under control of the direct needle control. In U.S. Patent Application Publication No. 2010/0012745, a substantial storage volume for intensifier fuel is also provided in the injector. See also U.S. Patent Application Publication No. 2008/0277504. These methods of operation of fuel injectors are highly efficient, but it has been discovered, can be improved.